As a background art of the field of the present technology, there is JP 2013-505485 A (PTL 1). This publication describes an optical modulator using a wafer bonding technology.
Further, WO 2011/092861 A (PTL 2) describes an optical element having an uneven side wall flattened in an atomic level, formed on a silicon substrate through a buried oxide film, and in which orientation of a surface of a silicon optical waveguide is a (110) plane and orientation of the side wall is a (111) plane, in order to provide a silicon optical waveguide with a less optical loss and without polarization dependency.
Further, “12.5-Gb/s Operation of Efficient Silicon Modulator Using Side-Wall Grating Waveguide” (NPL 1) discloses a Mach-Zehnder-type modulator including a side-wall grating waveguide.